Myanmar must hold a “proper investigation” into alleged atrocities against the Muslim Rohingya, a UN Security Council envoy said on Tuesday at the end of the highest-level diplomatic visit to the conflict area.

That campaign launched last August in the mainly Buddhist nation drove around 700,000 Rohingya refugees into Bangladesh.

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During the trip to Myanmar UN delegates met both civilian leader Aung San Suu Kyi and Senior General Min Aung Hlaing, who heads an army accused by the UN of “ethnic cleansing”.

“In order to have accountability there must be a proper investigation,” Britain’s UN ambassador Karen Pierce told reporters, after envoys visited both the Rohingya refugee camps in Bangladesh and Rakhine.

During his meeting late Monday with the UN envoys, the army chief denied his forces had committed rape and other sexual abuses during a crackdown which he ordered.

“The Tatmadaw [army] is always disciplined … and takes action against anyone who breaks the law,” he told the delegates, according to a posting late on Monday on his official Facebook page.

Rohingya women and girls in Bangladesh have provided consistent accounts of sexual violence – reports verified by conflict monitors – but Min Aung Hlaing said his forces have “no such history of sexual abuse.”

“It is unacceptable according to the culture and religion of our country,” he said, adding anyone found guilty of crimes would be punished.

He also repeated the official line that Myanmar was ready to take back the refugees who could be verified as residents, as per a repatriation deal with Bangladesh.

Several months after the deal was signed, no refugees have returned. They demand guarantees of safety, the right to return to their original villages and the granting of citizenship.

Another UN diplomat warned it will take “two or three years” for the refugees to be repatriated as the current time frame to implement the deal continues to slip.

“There is a need to speed up the process,” Mansour Ayyad Al-Otaibi, the Kuwaiti ambassador to the UN, said, adding conditions must be “safe and dignified” for return.

Bangladeshi accuses Myanmar of buying time by pretending to cooperate over repatriation for the benefit of the international community.

Myanmar says its neighbour has only handed back 8,000 repatriation forms so far, many of them incomplete, delaying the return process.

Myanmar denies the Rohingya citizenship and accompanying rights. It has driven out two thirds of its roughly 1.5 million Rohingya population since 2012.